Essentially Outlook 2007 introduced a limit to the number of objects/sessions it would support. This limit is 32, well, strictly it’s a 4 Kb block of memory. The processes and threads in Enterprise Vault use these sessions, and you can get odd connection errors when threads try to do MAPI activity as your approach and “break” the limit. You get an event like this one logged in the event log :

(That is taken from DTrace, but you can see the event log entry which is formed too)

In addition take note of the following :

Note: Each thread of a Mailbox Archiving task can hold two MAPI sessions (one for archiving and the other for synchronization) and hence setting the task to 5 threads/connections, actually means 10 profiles/sessions in the Outlook cache. Try to reduce the number of archiving threads to approximately 25 to allow for other threads, such as those from the Shopping Service and PST migration.

We have heard that Outlook 2010 removes this limit, however, it’s going to be some time before Outlook 2010 is certified to run on an EV Server!